The present invention relates to an electric power steering system for obtaining a steering assist force for vehicle by means of an electric motor.
Heretofore, there has been proposed the electric power steering system arranged as follows (see, for example, Japanese Unexamined Patent Publication No.2000-190855). Rotation of the electric motor is transmitted to a rotary element enclosing a rack shaft by means of a first bevel gear connected with an output shaft of the motor via a support shaft, and a second bevel gear meshed with the first bevel gear with their axes intersecting each other. Rotary motion of the rotary element is converted into linear motion of the rack shaft by means of a ball screw interposed between the rotary element and the rack shaft. The electric motor is driven into rotation in response to the rotation of a handle thereby supplying a steering assist force to a manual steering force for steering steerable road wheels coupled with the rack shaft.
Unfortunately, however, the conventional electric power steering system may sometimes encounter backlash at a meshing engagement area between the first and second bevel gears due to working precisions or assembly precisions of the first and second bevel gears. The backlash causes so-called rattling noises when a steering wheel is manipulated. The rattling noises may be conveyed to a passenger compartment to discomfort a vehicle operator and passenger.
Hence, there has been proposed an electric power steering system directed to the prevention of the backlash as follows. The electric motor is adapted for eccentric movement relative to a housing of the steering system. The eccentric movement of the electric motor provides adjustment of the position of the first bevel gear relative to the second bevel gear during the assembly of the system, thereby preventing the backlash (see, for example, Japanese Unexamined Patent Publication No.2000-190855).
However, it is quite difficult to avoid the backlash no matter how the adjustment is made during the assembly of the system as a countermeasure against the backlash, because tooth flanks of the bevel gear pair wear away from long-term use. Hence, there exists a need for cumbersome maintenance which includes periodical disassembly of the electric power steering system for backlash adjustment and the like.
As another countermeasure against the backlash, there is a common practice to interpose a shim between opposite surfaces of a support-shaft housing accommodating the support shaft and the first bevel gear and the electric motor in end-to-end relation such that the support shaft and first bevel gear may be axially moved in unison with the output shaft of the electric motor thereby adjusting the degree of meshing engagement between the first bevel gear and the second bevel gear.
In the above operation for backlash adjustment, however, the shimming must be usually repeated over and over again because a proper meshing engagement between these bevel gears cannot be achieved by a single shimming. Furthermore, each shimming involves disassembling the electric motor from the support-shaft housing and assembling the electric motor thereto. This makes the backlash adjustment complicated and cumbersome. In this approach, as well, the tooth flanks of these bevel gears wear away from long-term use of the system and hence, the occurrence of backlash is inevitable.
In addition, if the shimming involves a more than necessary amount of movement of the first bevel gear relative to the second bevel gear, a meshing engagement point between these bevel gears is shifted far away from a predetermined meshing engagement point. This results in the decrease of power transmission efficiency between these bevel gears.
There is another problem that the rotation of the support shaft entails vibrations, which propagate through a bearing to the support-shaft housing to produce noises.
It is an object of the invention to provide an electric power steering system capable of preventing the backlash over an extended period of time.
It is another object of the invention to provide an electric power steering system adapted to facilitate the backlash adjustment.
It is still another object of the invention to provide an electric power steering system adapted to prevent the backlash adjustment from entailing a significant displacement of the meshing engagement point between the first and second bevel gears.
It is yet another object of the invention to provide an electric power steering system essentially obviating the necessity for the backlash adjustment.
It is yet another object of the invention to provide an electric power steering system designed to suppress noises caused by the vibrations of the support shaft.
An electric power steering system according to a first aspect of the invention comprises: a first bevel gear driven into rotation by an electric motor; a second bevel gear meshed with the first bevel gear with their axes intersecting each other and allowed to move toward the first bevel gear; a biasing member for biasing the second bevel gear toward the first bevel gear; a rack shaft linearly moved thereby steering steerable road wheels; a rotary element rotatably supported by a rack housing via a rolling bearing as enclosing the rack shaft and operating to transmit the rotation of the electric motor via the first bevel gear and the second bevel gear; and a power conversion mechanism formed between the rotary element and the rack shaft for converting the rotary motion of the rotary element to the linear motion of the rack shaft.
According to the electric power steering system of the above construction, even if the bevel gears wear away from long-term use, the second bevel gear is biased by the biasing member into movement toward the first bevel gear as following the wearing away of the gears. This leads to the prevention of the backlash at the meshing engagement area between these bevel gears. Thus, the system does not require the maintenance for preventing the rattling noises and ensures over an extended period of time that the vehicle operator and passenger are not discomforted by the rattling noises.
The electric power steering system may preferably be arranged such that the biasing member is interposed between an outer lateral surface of an outer ring of the rolling bearing and a biasing member seat of the housing opposite from the outer lateral surface, thereby moving the second bevel gear toward the first bevel gear in conjunction with the rotary element.
In this case, the biasing member biases the outer ring of the rolling bearing thereby applying a biasing force to the second bevel gear via the rotary element and moving the second bevel gear toward the first bevel gear in conjunction with the rotary element. This obviates the necessity for a bearing for permitting relative rotation between the biasing member and the rotary element so that the structure of the system is simplified. In this mode, it is preferred that the rack housing comprises a first housing for supporting one end of the rotary element as allowing for the relative axial movement thereof, and a second housing for supporting the other end of the rotary element as inhibiting the relative axial movement thereof, and that a housing adjuster provides for adjustment of the relative axial positions of the first housing and the second housing with respect to the rack shaft. In this mode, the housing adjuster provides the adjustment of the relative axial positions of the first and second housings, thereby providing adjustment of a gap between the outer lateral surface of the outer ring of the rolling bearing and the biasing member seat of the housing opposite from the outer lateral surface of the outer ring. The gap adjustment provides for adjustment of the biasing force on the second bevel gear applied by the biasing member. Thus, the backlash adjustment is facilitated.
An electric power steering system according to a second aspect of the invention comprises: a support shaft integrally rotatably connected with an output shaft of an electric motor; a first bevel gear provided at a distal end of the support shaft; a support-shaft housing accommodating the support shaft and the first bevel gear; a rack shaft linearly moved thereby steering steerable road wheels; a rack housing for supporting the rack shaft as allowing for the linear movement thereof; a second bevel gear meshed with the first bevel gear with their axes intersecting each other; a rotary element rotatably supported by the rack housing as enclosing the rack shaft and operating to transmit the rotation of the electric motor via the first bevel gear and the second bevel gear; a power conversion mechanism formed between the rotary element and the rack shaft for converting the rotary motion of the rotary element to the linear motion of the rack shaft; a retaining member mounted to the support-shaft housing as allowed to move axially of the support shaft for rotatably retaining the support shaft via a bearing; and a retaining member adjuster for adjustment of the position of the retaining member relative to the support-shaft housing with respect to an axial direction of the support shaft.
According to the electric power steering system of the second aspect hereof, the retaining member adjuster provides the adjustment of the position of the retaining member relative to the support-shaft housing with respect to the axial direction of the support shaft, thereby providing the axial movement of the first bevel gear along with the support shaft. Thus, the backlash adjustment for the first and second bevel gears can be done without performing the conventional shimming. As a result, the backlash adjustment is accomplished easily and quickly.
The electric power steering system of the second aspect hereof may be arranged such that a proximal end portion of the support shaft is retained by the retaining member via the bearing, whereas a distal end portion of the support shaft is retained in a manner to be movable toward the second bevel gear and is biased toward the second bevel gear by a biasing member.
In this case, even if the bevel gears wear away from long-term use, the first bevel gear is biased by the biasing member into movement toward the second bevel gear as following the wearing away of the bevel gears. Hence, the wearing away of the bevel gears is prevented from causing the backlash at the meshing engagement area therebetween. Thus, the system does not require the maintenance for preventing the rattling noises, ensuring over an extended period of time that the vehicle operator and passenger are not discomforted by the rattling noises. In this mode, the system may further comprise a joint for pivotally connecting the proximal end portion of the support shaft with the output shaft of the electric motor. This mode is more effective to prevent the backlash because the joint allows for a smooth movement of the first bevel gear toward the second bevel gear.
The electric power steering system of the second aspect hereof may preferably be arranged such that the rack housing comprises a first housing for supporting one end of the rotary element as allowing for the relative axial movement thereof, and a second housing for supporting the other end of the rotary element as inhibiting the relative axial movement thereof, and that a housing adjuster provides for adjustment of the relative axial positions of the first housing and the second housing with respect to the rack shaft.
In this case, the housing adjuster provides the adjustment of the relative axial positions of the first and second housings with respect to the rack shaft, thereby providing the axial movement of the rotary element along with the second bevel gear with respect to the rack shaft for accomplishing adjustment of a meshing engagement point between these bevel gears. Thus is prevented the decrease in power transmission efficiency between these bevel gears.
An electric power steering system according to a third aspect of the invention comprises: a support shaft integrally rotatably connected with an output shaft of an electric motor; a first bevel gear provided at a distal end of the support shaft; a second bevel gear meshed with the first bevel gear with their axes intersecting each other; a rack shaft linearly moved thereby steering steerable road wheels; a rotary element rotatably supported by the rack housing as enclosing the rack shaft and operating to transmit the rotation of the electric motor via the first bevel gear and the second bevel gear; and a power conversion mechanism formed between the rotary element and the rack shaft for converting the rotary motion of the rotary element to the linear motion of the rack shaft; wherein the support shaft is allowed to move toward the second bevel gear and is biased toward the second bevel gear by a biasing member.
According to the electric power steering system of the third aspect hereof, the first bevel gear can be pressed against the second bevel gear because the support shaft is movable toward the second bevel gear and is biased toward the second bevel gear by the biasing member. This essentially negates the need for the backlash adjustment in the system assembly procedure. Even if the bevel gears wear away from long-term use, the first bevel gear is biased by the biasing member into movement toward the second bevel gear as following the wearing away of the bevel gears. This leads to the prevention of the backlash at the meshing engagement area between these bevel gears. Thus, the system does not require the maintenance for preventing the rattling noises, ensuring over an extended period of time that the vehicle operator and passenger are not discomforted by the rattling noises.
The electric power steering system of the third aspect hereof may preferably be arranged such that the biasing member is interposed between the support shaft and the output shaft of the electric motor. This facilitates the mounting of the biasing member.
The electric power steering system of the third aspect hereof may preferably be arranged such that an elastically deformable buffer member is interposed between the support shaft and a support-shaft housing supporting the support shaft. In this case, the buffer member is capable of absorbing vibrations caused by the rotation of the support shaft, thus suppressing noises attributable to the vibrations.
The electric power steering system of the third aspect hereof may preferably be arranged such that the rack housing comprises a first housing for supporting one end of the rotary element as allowing for the relative axial movement thereof, and a second housing for supporting the other end of the rotary element as inhibiting the relative axial movement thereof, and that a housing adjuster provides for adjustment of the relative axial positions of the first housing and the second housing with respect to the rack shaft.
In this case, the housing adjuster provides the adjustment of the relative axial positions of the first and second housings with respect to the rack shaft, thereby providing the axial movement of the rotary element along with the second bevel gear with respect to the rack shaft for accomplishing adjustment of a meshing engagement point between these bevel gears. Thus is prevented the decrease in power transmission efficiency between these bevel gears.